2.2.30 [HTML5:2014] Section 4.11.4.2 Serializing bitmaps to a file

C0045: Many image formats other than PNG are supported

The specification states:

 ... Serializing bitmaps to a file
     ...
     User agents must support PNG ("image/png"). User agents may support other types. If 
     the user agent does not support the requested type, it must create the file using the 
     PNG format.

All document modes (All versions)

Many image formats other than PNG are supported (gif, jpeg, ico, bmp, etc.).